An updated overview of e cigarette impact on human health Respiratory Research Springer Nature Link Propylene glycol, a component of electronic cigarette liquid, damages epithelial cells in human small airways Respiratory Research Springer Nature Link All E Cigarettes Emit Harmful Chemicals, but Some Emit More Than Others Berkeley Lab Berkeley Lab News Center Researchers at the University of California, Riverside, found that heating propylene glycol, the main ingredient in most e cigarette liquids, can produce two toxic chemicals, methylglyoxal and acetaldehyde, which damage lung cells. Using E cigarette, or Vaping Product, Use Associated Lung Injury (EVALI) Fact Sheets Yale Medicine